The Spin Zone

Black holes have only 3 properties: mass, charge and spin. Astronomers have enough trouble measuring the masses of black holes; measuring charge and spin are even more difficult. How can we measure the spin of something which lacks a solid surface? One way is to measure the effects of that spin on matter near the spinning black hole. A spinning black hole is expected to actually drag space and time along with it, to set space and time spinning. Matter located in this spacetime should show predictable effects of being in this "spinning spacetime". These effects are illustrated in the image above, which compares a non-spinning black hole on the left to a spinning black hole on the right. The upper images show the disk of matter being accreted by the black hole. In the non-spinning case, matter falls into the black hole from a relatively far distance, but in the spinning black hole case shown in the upper left, matter can orbit safely much closer to the black hole's event horizon. The lower panels show the effects of spin on X-rays emitted by fast moving, hot iron atoms near the black hole. For iron atoms near a spinning black holes, the emitted X-rays are shifted to lower X-ray energies. This "iron" effect has been detected in a number of black holes by the Chandra and the XMM-Newton X-ray observatories. These observations show that some iron emission is more effected than others; evidently not all black holes spin at the same rate.